A Queens computer whiz helped scam an estimated $400,000 from dozens of victims by posing as a Microsoft Windows expert, law-enforcement sources said.

Julian Alexander, 56, a self-proclaimed marketing entrepreneur for small businesses, concocted his slick ruse with the help of a “co-conspirator” in India and used it to victimize about 90 New Yorkers between the ages of 60 and 80, the sources said.

Police investigators caught up with Alexander after many of his victims filed complaints and cops traced his digital footprint, said a source, describing him as “very adept at using a computer for fraudulent means.”

Alexander of Jamaica was arrested Thursday by the FBI-NYPD Cyber Task Force on charges of grand larceny and unauthorized use of a computer.

Alexander and his overseas accomplice would first send a bogus e-mail that contained a “pop up” claiming that the user’s computer had a virus, sources said.

The duo would follow up with a phone call explaining that they worked for “Windows,” without saying the word, “Microsoft,” sources said.

They would then read from a prepared script saying they were trained Windows specialists who could easily remove the virus for $200, sources said.

They would ask the victims for remote access to their computers and request payment for the fake services, sources said.

The unsuspecting victims complied, leaving their online bank accounts open. The crooks would then transfer the victims’ cash into their own accounts, sources said.